Existing Buildings and the 10% Rule 1

[PEFLWI]

My question relates to the so called 10% rule in the IBC for Existing Buildings. Is the intent to allow alterations to a structure that increase in the wind or seismic loads by 10% without having to reinforce the structure? In my particular case the existing structure was built in the 1960s. Using the current version of the code the structure is significantly overstressed without any alterations. The alterations being considered would make it worse, but not by more than 10%. My view is that if a member is stressed to 160% of its capacity using the current codes, than the 10% rule does not apply. I have included the relevant section of the Existing Building Code for reference.

Except as permitted by Section 807.6, where the alteration increases design lateral loads, or where the alteration results in prohibited structural irregularity as defined in ASCE 7, or where the alteration decreases the capacity of any existing lateral load-carrying structural element, the structure of the altered building or structure shall be shown to meet the wind and seismic provisions of the Florida Building Code, Building. Reduced seismic forces shall be permitted.

Exception: Any existing lateral load-carrying structural element whose demand-capacity ratio with the alteration considered is not more than 10 percent greater than its demand-capacity ratio with the alteration ignored shall be permitted to remain unaltered.

 

[straub46]

...structural element whose demand-capacity ratio with the alteration considered is not more than 10 percent greater than its demand-capacity ratio with alteration ignored...

I've always treated this as you compare your stress ratios with the alteration considered vs not and see what the difference is. i.e. with the alteration it's at 1.65 and without it's at 1.60, then it would be ok to make the alteration without upgrading the whole system, since that's a 5% increase.

Note your comparisons need to be made with current wind speed pressures, etc, and needs to be compared to the original design. If there were previous alterations that didn't address the LFRS, then this alteration may be less than 10% of what's there now (after the previous alteration), but may push it over 10% of the original design.

Go Bucks!

 

[JLNJ]

1.65 is just 3.125% greater than 1.60.

 

[Rabbit12]

I'm with straub. I compare apples to apples (same code) and look at unity values.

As JLNJ points out 1.65 is 3.125% above 1.6 but I sure wouldn't go to 1.76 unity which is 10%. I'd max out at 1.7 so essentially treating the in-situ condition as at unity.

With today's codes the existing structure may not be below unity. However, it's a tough sell to tell a client his structure is wildly overstressed when it's been performing flawlessly for many many years.

 

[Skinnattittar]

I think straub46 has the correct interpretation of the code, but I would say if the system is significantly over stressed, it would be irresponsible to not address it in the planned addition/renovations.

If we were talking about a difference of minor percentages over capacity, then an in depth analysis of the structural elements (interior walls sheathed in gypsum do add some rigidity, dead loads, chimney stacks, etc...) and comparing it to the failure strength will usually prove the system at least safe during a storm event, but if you're over capacity by 60%, then something definitely should be done or what's the point of anything being designed sufficiently? Other than to say "hey, I did my part correctly, it was the other guy 50 years ago who didn't do their job, its not on me to fix their mistakes."

 

[StrucPEng]

A recent article in STRUCTURE Magazine on this with some code interpretation and examples.

https://www.structuremag.org/?p=17307

 

[PEFLWI]

StrucPEng:

The article you linked to if for the 5% overstress. Is there a similar article for the 10% for wind?

Thanks.

 

[Rabbit12]

It's the exact same concept. If you are making changes that increases stresses to a lateral force resisting element by more than 10% you need to analyze the affected members and make sure they are adequate.

I'd be quite leery of your 60% overstress. Are you sure you've sharpened your pencil and have the loads correct? Are all members sizes and geometry correct?

 

[WARose]

One thing I'd like to point out in this (at the risk of getting on a tangent) is: be careful how many time this 5-10% rule has been used. In other words, know what it was originally designed for (if you can). I say this as someone who has done a lot of industrial structures where 5 years after it was completed, a guy came along and added something and used a similar rule. Then 5-10 years later someone added more (not realizing what was done 5 years before).....and on it goes.....and then I show up and realize what has happened and modifications are needed. (Also in part because of the newer seismic standards.)

So it is just something to watch for. (In case this structure is industrial.)

 

[Rabbit12]

WARose is 100% correct. That 5%/10% practice is common in the industrial world.